Wireless communication device, wireless communication system, wireless communication method, and program

ABSTRACT

There is provided a wireless communication device including a communication unit that transmits a reference signal, a first multiplication unit that performs multiplication of first transmission weight that is determined based on reception of the reference signal by a communication partner, and a second multiplication unit that performs multiplication of second transmission weight that is determined based on reception of the reference signal by the communication partner. The communication unit transmits a reference signal with weight that is obtained by multiplying the reference signal by the first transmission weight after determination of the first transmission weight.

TECHNICAL FIELD

The present disclosure relates to a wireless communication device, awireless communication system, a wireless communication method, and aprogram.

BACKGROUND ART

Currently, in third generation partnership project (3GPP),standardization of a wireless communication system for 4G has beenpromoted. In 4G, a technology such as relay, carrier aggregation,coordinated multiple point transmission and reception (CoMP), and multiuser multi input multi output (MU-MIMO) has been attracting attention.

Relay is considered as an important technology to improve a throughputof a cell edge. In addition, carrier aggregation is a technology thatcan handle a band width of 20 MHz×5=100 MHz by handling, for example,five frequency bands each having a band width of 20 MHz, together. Bysuch carrier aggregation, improvement of a maximum throughput can beexpected.

In addition, CoMP is a technology in which a plurality of base stationstransmit and receive data in cooperation in order to improve coverage ofa high data rate. In addition, MU-MIMO is a technology that improves asystem throughput so that a plurality of users use a resource block ofthe same frequency and the same time, on which spatial multiplexing isperformed. As described above, further improvement of the performance in4G (LTE-Advanced) by various technologies has been discussed.

Here, MU-MIMO is described in detail. In 3.9G (LTE), there aretechnologies of MU-MIMO and single user MINO (SU-MIMO). For example, asdiscussed in Patent literature 1, SU-MIMO is a technology in which aplurality of channels are used so that single user equipment (UE)performs spatial multiplexing of the plurality of channels althoughspatial multiplexing is not performed between pieces of UE.

On the other hand, as described above, MU-MIMO is a technology in whicheach UE uses a resource block of the same frequency and the same time,on which spatial multiplexing is performed (spatial multiplexing isperformed between pieces of UE). However, in MU-MIMO that is realized in3.9G, each UE handles a mere single channel. On the contrary, in 4G,MU-MIMO in which each UE can handle a plurality of channels is beingrealized.

In order to achieve such MU-MIMO in 4G, it has been studied that twotypes (V1 and V2) of transmission weight are used in a base station. TheV1 is transmission weight that realizes directivity, and the V2 istransmission non-directional weight, the main purpose of which is toadjust a phase. The V1 and V2 can be determined, for example, in UE. Tobe more specific, the UE receives a reference signal that is transmittedfrom a base station, obtains a channel matrix H from the receptionresult of the reference signal, and determines optimal V1 and V2 for thechannel matrix H.

CITATION LIST Patent Literature

Patent Literature 1: JP 2005-184730A

SUMMARY OF INVENTION Technical Problem

However, high calculation load in UE for determining transmission weightV1 and transmission weight V2 is concerned because the transmissionweight V1 and transmission weight V2 are complex numbers.

Therefore, in the present disclosure, there are proposed a new andimproved wireless communication device, wireless communication system,wireless communication method, and program that can suppress calculationload in a communication partner for determining transmission weight.

Solution to Problem

According to an embodiment of the present disclosure, there is provideda wireless communication device including a communication unit thattransmits a reference signal, a first multiplication unit that performsmultiplication of first transmission weight that is determined based onreception of the reference signal by a communication partner, and asecond multiplication unit that performs multiplication of secondtransmission weight that is determined based on reception of thereference signal by the communication partner. The communication unittransmits a reference signal with weight that is obtained by multiplyingthe reference signal by the first transmission weight afterdetermination of the first transmission weight.

The wireless communication device may further includes a referencesignal management unit that manages a resource for transmitting thereference signal with weight.

The reference signal management unit may allocate a resource fortransmitting the reference signal with weight and a resource fortransmitting the reference signal after determination of the firsttransmission weight.

The reference signal management unit may allocate more resources fortransmitting the reference signal than the resource for transmitting thereference signal with weight.

The reference signal management unit may allocate a resource so thattransmission frequency of the reference signal on a time axis becomeshigher than transmission frequency of the reference signal with weighton the time axis.

The reference signal management unit may allocate a resource so that adensity of a resource for transmitting the reference signal on afrequency axis becomes higher than a density of a resource fortransmitting the reference signal with weight on the frequency axis.

The wireless communication device may further include a scheduler thatallocates a resource for communication of a first scheme or a secondscheme to each communication partner. The scheduler may allocate aresource within a first frequency range for the communication of thefirst scheme, and allocate a resource within a second frequency rangefor the communication of the second scheme.

The first frequency range may be a frequency range to which a resourcefor transmitting the reference signal with weight is allocated. Thesecond frequency range may be a frequency range to which a resource fortransmitting the reference signal is allocated.

The first scheme may be multi user multi input multi output (MU-MIMO),the second scheme may be single user multi input multi output (SU-MIMO).

The wireless communication device may further include a scheduler thatallocates a resource for communication of a first scheme or a secondscheme to each communication partner. The scheduler may allocate, forthe communication of the first scheme, a resource within a frequencyrange to which a resource for transmitting the reference signal withweight is allocated, and allocate, for the communication of the firstscheme or the second scheme, a resource within a frequency range towhich a resource for transmitting the reference signal is allocated.

Update frequency of the second transmission weight may be higher thanupdate frequency of the first transmission weight.

The first transmission weight may be weight for forming directivity, andthe second transmission weight may be non-directional weight foradjusting a phase.

Further, according to another embodiment of the present disclosure,there is provided a program for causing a computer to function as awireless communication device that includes a communication unit thattransmits a reference signal, a first multiplication unit that performsmultiplication of first transmission weight that is determined based onreception of the reference signal by a communication partner, and asecond multiplication unit that performs multiplication of secondtransmission weight that is determined based on reception of thereference signal by the communication partner. The communication unitmay transmit a reference signal with weight that is obtained bymultiplying the reference signal by the first transmission weight afterdetermination of the first transmission weight.

Further, according to another embodiment of the present disclosure,there is provided a wireless communication method including transmittinga reference signal, multiplying the reference signal by firsttransmission weight that is determined based on reception of thereference signal by a communication partner, and transmitting areference signal with weight that is obtained by multiplying thereference signal by the first transmission weight.

Further, according to another embodiment of the present disclosure,there is provided a wireless communication system including a firstwireless communication device, and a second wireless communicationdevice that includes, a communication unit that transmits a referencesignal, a first multiplication unit that performs multiplication offirst transmission weight that is determined based on reception of thereference signal by the first wireless communication device, and asecond multiplication unit that performs multiplication of secondtransmission weight that is determined based on reception of thereference signal by the first wireless communication device. Thecommunication unit transmits a reference signal with weight that isobtained by multiplying the reference signal by the first transmissionweight after determination of the first transmission weight.

Further, according to another embodiment of the present disclosure,there is provided a wireless communication device including acommunication unit that receives a reference signal from a communicationpartner, and a weight determination unit that determines firsttransmission weight and second transmission weight based on a receptionresult of the reference signal by the communication unit. When areference signal with weight that is obtained by multiplying thereference signal by the first transmission weight is received by thecommunication unit, the weight determination unit determines the secondtransmission weight based on a reception result of the reference signalwith weight.

Further, according to another embodiment of the present disclosure,there is provided a wireless communication device including a schedulerthat allocates a resource for communication of a first scheme or asecond scheme to each communication partner. The scheduler allocates aresource within a first frequency range for the communication of thefirst scheme, and allocates a resource within a second frequency rangefor the communication of the second scheme.

The first scheme may be multi user multi input multi output (MU-MIMO),and the second scheme may be single user multi input multi output(SU-MIMO).

Advantageous Effects of Invention

As described above, according to the present disclosure, calculationload in a communication partner for determining transmission weight canbe suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustrative diagram illustrating a configuration of awireless communication system according to an embodiment of the presentdisclosure.

FIG. 2 is an illustrative diagram illustrating an example ofmultiplication order of transmission weight.

FIG. 3 is an illustrative diagram illustrating relationship of V1 andV2.

FIG. 4 is an illustrative diagram illustrating a determination methodusing a comparative example of the transmission weight V1 andtransmission weight V2_MU.

FIG. 5 is an illustrative diagram illustrating a determination methodusing a comparative example of transmission weight in a case in whichMU-MIMO and SU-MIMO are present.

FIG. 6 is an illustrative diagram illustrating a configuration of a basestation according to an embodiment of the present disclosure.

FIG. 7 is an illustrative diagram illustrating of a configuration of aweight multiplication unit.

FIG. 8 is an illustrative diagram illustrating a configuration of aweight multiplication unit according to a variant.

FIG. 9 is an illustrative diagram illustrating a configuration of amobile station according to an embodiment.

FIG. 10 is an illustrative diagram illustrating a first embodiment ofthe present disclosure.

FIG. 11 is an illustrative diagram illustrating a second embodiment ofthe present disclosure.

FIG. 12 is an illustrative diagram illustrating a third embodiment ofthe present disclosure.

FIG. 13 is an illustrative diagram illustrating a resource allocationexample of a V1*CSI_RS and a CSI_RS according to a fourth embodiment.

FIG. 14 is an illustrative diagram illustrating a specific example ofresource allocation according to a fifth embodiment.

FIG. 15 is an illustrative diagram illustrating a specific example ofresource allocation according to a sixth embodiment.

FIG. 16 is an illustrative diagram illustrating a specific example ofresource allocation according to a seventh embodiment.

FIG. 17 is a flowchart illustrating an operation of a base stationaccording to the embodiments of the present disclosure.

FIG. 18 is a flowchart illustrating an operation of a mobile stationaccording to the embodiments of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the drawings, elements that have substantiallythe same function and structure are denoted with the same referencesigns, and repeated explanation is omitted.

In addition, in this specification and drawings, a plurality of elementshaving substantially the same function and structure may bedistinguished so as to be denoted with different alphabets after thesame reference numeral. For example, a plurality of configurationshaving substantially the same function and structure such as mobilestations 20A, 20B, and 20C may be distinguished as appropriate. However,when there is no particular need to distinguish a plurality of elementshaving substantially the same function and structure individually, theplurality of elements are denoted with the mere same reference numeral.For example, when there is no particular need to distinguish mobilestations 20A, 20B, and 20C, the mobile stations are merely referred toas mobile station 20.

In addition, “Description of Embodiments” is made in accordance with theorder of the following items.

-   -   1. Outline of a wireless communication system        -   1-1. Configuration of the wireless communication system        -   1-2. Transmission weight (V1 and V2)        -   1-3. Feedback scheme of transmission weight        -   1-4. Dynamic switching        -   1-5. Comparative example    -   2. Basic configuration of a base station    -   3. Basic configuration of a mobile station    -   4. Description of each embodiment        -   4-1. First embodiment        -   4-2. Second embodiment        -   4-3. Third embodiment        -   4-4. Fourth embodiment        -   4-5. Fifth embodiment        -   4-6. Sixth embodiment        -   4-7. Seventh embodiment    -   5. Operation of the base station and the mobile station    -   6. Conclusion

1. Outline of a Wireless Communication System

Currently, in 3GPP, standardization of a wireless communication systemfor 4G has been promoted. An embodiment of the present disclosure can beapplied to the wireless communication system for 4G as an example, and,first, the outline of the wireless communication system for 4G isdescribed.

1-1. Configuration of a Wireless Communication System

FIG. 1 is an illustrative diagram illustrating a configuration of awireless communication system 1 according to an embodiment of thepresent disclosure. As illustrated in FIG. 1, the wireless communicationsystem 1 according to the embodiment of the present disclosure includesa base station 10 and a plurality of mobile stations 20. Note that thebase station 10 may a wireless communication device such as eNodeB, arelay node, or home eNodeB that is a household small base station in 4G.In addition, the mobile station 20 may be a wireless communicationdevice such as a relay node or UE in 4G.

The base station 10 controls communication with the mobile station 20 ina cell. In addition, the base station 10 is operated using three sectorsso that each of the sectors has, for example, an angle of 120 degrees asillustrated in FIG. 1. In addition, the base station 10 includes aplurality of antennas, and can form directivity in a plurality ofdirections in each of the sectors (four directions in the exampleillustrated in FIG. 1) by multiplying a transmission signal from each ofthe antennas by transmission weight V1 that is described later.

Therefore, the base station 10 can perform multiplexing so that mobilestations 20A and 20B that exist in different directions when viewed fromthe base station 10 are spatially separated. That is, the base station10 can communicate with the plurality of the mobile stations 20 byMU-MIMO. Note that the base station 10 can also communicate with themobile stations 20 by SU-MIMO.

The mobile station 20 is a wireless communication device thatcommunicates with the base station 10 by MU-MIMO or SU-MIMO. The mobilestation 20 moves in accordance with the movement of a moving body suchas a user and a vehicle. Note that, in the embodiment, the mobilestation 20 is described as an example of a wireless communication devicethat wirelessly communicates with the base station 10, and theembodiment can be also applied to a wireless communication device thatis installed in a fixed manner.

1-2. Transmission Weight (V1 and V2)

In 4G in the realization of the MU-MIMO, it is has been studied thattransmission weight that is referred to as V2 is used in addition to theV1 that is described above (double codebook scheme). The V1 istransmission weight that realizes directivity as described above. SuchV1 has a characteristic such as coverage of a wide frequency area andlower update frequency than that of the V2.

On the other hand, the V2 is transmission non-directional weight, themain purpose of which is to adjust a phase. More specifically, the V2 isused for maximizing reception power by adjusting a phase of each pathbetween antennas of the mobile station 20 and the base station 10. Inaddition, the V2 has a characteristic such as coverage of a narrowfrequency area and higher update frequency than that of the V1.

The base station 10 according to the embodiment realizes MU-MIMO bymultiplying transmission data by such transmission weight V1 andtransmission weight V2. Note that, as illustrated in FIG. 2, the basestation 10 may multiply transmission data by transmission weight inorder of V2 and V1, and may multiply transmission data by transmissionweight in order of V1 and V2.

FIG. 3 is an illustrative diagram illustrating a relationship of V1 andV2. As illustrated in FIG. 3, when the base station 10 includes 8antennas, these antennas operate as two set of linear array antennas 4Aand 4B each of which is constituted of four elements. Note that thelinear array antennas 4A and 4B operate as array antennas having thesame directivity as illustrated in FIG. 3.

In addition, the V2 operates so that two code words of transmission dataare distributed into the two set of linear array antennas 4A and 4B bychanging the phase. That is, the V2 operates so as to change the phaseof a transmission signal to be supplied to the linear array antennas 4Aand 4B that perform transmission in the same direction. On the otherhand, the V1 is applied to each antenna as illustrated in FIG. 3 andoperates so that the linear array antennas 4A and 4B form directivity.

Specific examples of the above-described V1 and V2 are described below.Note that “d” in “Formula 1” that represents the V1 indicates a distancefrom a reference antenna, “λ” indicates a wavelength, “θ” indicates adirection of beam, and “i” indicates an antenna number. In addition, “H”in “Formula 2” that represents V2 indicates a channel matrix.

$\begin{matrix}{{V\; 1(i)} = \begin{bmatrix}1 \\{\exp \left( {{- j}\; 2{\pi/\lambda}*d\; 1\sin \; {\theta (i)}} \right)} \\{\exp \left( {{- j}\; 2{\pi/\lambda}*d\; 2\sin \; {\theta (i)}} \right)} \\{\exp \left( {{- j}\; 2{\pi/\lambda}*d\; 3\sin \; {\theta (i)}} \right)}\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \\{{{V\; 2} = \begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}},\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

As illustrated in “Formula 2”, the V2 is transmission weight that isrepresented as plus or minus 1, or plus or minus j. Note that the jindicates an imaginary number. Thus, a load for multiplying a certainmatrix by the V2 is small. On the other hand, the V1 is transmissionweight that is described by a directional vector, and is not a matrixthat is represented by plus or minus 1 and plus or minus j. Therefore,in calculation using the V1, calculation load is increased.

Note that when transmission data of the base station 10 is “S” andreception data of the mobile station 20 is “R”, the reception data R ofthe mobile station 20 can be represented as the following “Formula 3” or“Formula 4”.

R=H·V1·V2·S   [Math. 3]

R=H·V2·V1·S   [Math. 4]

1-3. Feedback Scheme of Transmission Weight

As a feedback scheme of MIMO for determining the above-describedtransmission weight V1 and transmission weight V2, three schemes ofimplicit feedback, explicit feedback, and SRS-based feedback areconceivable. In 4G, as a feedback scheme of MIMO for determining thetransmission weight V1 and transmission weight V2, the use of theimplicit feedback is determined because a load on a feedback circuit issmall. For reference, each of the feedback schemes in 3.9G (LTE) isdescribed below.

(1) Implicit Feedback

In a base station, 16 types of transmission weight (V1) to transmissionweight (V16) are prepared (pre-coded) for a coodbook that has beendesigned in beforehand. A mobile station that receives a referencesignal from the base station obtains a channel matrix H between the basestation and mobile station. In addition, the mobile stationpre-determines HV having the highest reception power from among HV(1),HV(2), . . . , HV(16). After that, the mobile station provides feedbackof an index number that indicates V that makes reception power maximum,to the base station. The base station transmits data using the Vcorresponding to the index that is fed back.

(2) Explicit Feedback

The base station transmits a reference signal, and the mobile stationthat receives the reference signal from the base station obtains achannel matrix H between the base station and the mobile stationsimilarly to the case of the implicit feedback. In addition, the mobilestation provides feedback of the channel matrix H as-is, to the basestation. The base station calculates and creates a desired transmissionweight from the channel matrix H in downlink that is fed back from themobile station. In addition, the base station transmits data using thecreated transmission weight. In this explicit feedback, there is aproblem that a resource that is used for feedback becomes larger thanthat of the implicit feedback because a channel matrix H is transmittedas-is at the time of feedback.

(3) SRS-Based Feedback

The mobile station transmits a reference signal, and the base stationthat receives the reference signal from the mobile station obtains achannel matrix in uplink between the mobile station and the basestation. When the reversibility of a channel can be established (in acase of a TDD mode), the base station can makes a virtual channel matrixin downlink from the channel matrix. A scheme in which a virtual channelmatrix in downlink is made as described above is the SRS-based feedback.In the SRS-based feedback, there is a problem such that, whencalibration is not performed in which variations of analog circuits inthe base station are compensated, the reversibility of channels inuplink and downlink (channel matrix that includes a characteristic ofthe analog circuit) is not established.

1-4. Dynamic Switching

In 4G (LTE-Advanced), it is has been studied that setting of MIMO isdynamically switched between MU-MIMO and SU-MIMO. In addition, inMU-MIMO in 4G, the use of eight streams has been studied. In the case ofeight streams, one matrix for phase adjustment of V2 as described in“1-2. Transmission weight (V1 and V2)” is used.

The example is described above in which MU-MIMO is realized by combiningV1 having a 4×4 matrix and V2 having a 2×2 matrix. On the other hand,mere V2 having an 8×8 matrix is used for SU-MIMO. In addition, eachelement of the V2 having the 8×8 matrix is represented by the plus orminus 1 and plus or minus j, similarly to the V2 having the 2×2 matrix.Note that j indicates an imaginary number.

As described above, different V2 are used for MU-MIMO and SU-MIMO, and,in this specification, V2 for MU-MIMO is referred to as V2_MU, andweight for SU-MIMO is referred to as V2_SU, thereby distinguishing thetwo of V2.

1-5. Comparative Example

In 4G and the embodiments, as described in “1-3. Feedback scheme oftransmission weight,” the transmission weight V1 and transmission weightV2_MU are determined by implicit feedback. Here, in order to clarify thetechnical significance of the embodiments, a determination method usinga comparative example of the transmission weight V1 and transmissionweight V2_MU is described with reference to FIG. 4.

FIG. 4 is an illustrative diagram illustrating the determination methodusing a comparative example of the transmission weight V1 andtransmission weight V2_MU. In FIG. 4, the horizontal axis indicates atime. In addition, CSI indicates a channel state information referencesignal (CSI_RS).

As illustrated in FIG. 4, the base station transmits a CSI_RS (step 1),and the mobile station obtains a channel matrix H from the CSI_RSreceived from the base station. In addition, the mobile stationevaluates optimal V1 for the obtained channel matrix H, among four typesof V1 candidate. For example, the mobile station selects V1 that makesreception power maximum, among four types of V1 candidate. In addition,the mobile station evaluates and selects optimal V2_MU. After that, themobile station provides feedback of Index_V1 that indicates the selectedV1 and Index_V2 that indicates V2_MU to the base station (step 2). Thebase station determines V1 and V2_MU on the basis of the feedback fromthe mobile station.

When the base station and the mobile station determines V1 and V2_MU,the base station and the mobile station updates the only V2_MU multipletimes (step 3) followed by updating the V1 and V2_MU (step 4). Asdescribed above, update frequency of V2_MU is higher than updatefrequency of V1.

Here, the mobile station performs calculation using a plurality of typesof V1 when the mobile station selects V1. As described in “1-2.Transmission weight (V1 and V2)”, load of the mobile station in the caseof selecting V1 becomes large because load of calculation using V1 islarger than load of calculation using V2₁₃ MU.

On the other hand, it is conceived that calculation using V1 is notdesired in the case of selecting V2. However, the idea is wrong, and themobile station performs calculation using V1 in the case of selectingV2. This is because the mobile station obtains a channel matrix H from anewly received CSI_RS, multiplies the channel matrix H by alreadydetermined V1, and evaluates optimal V2_MU for the channel matrix H thatis multiplied by the V1. As described above, in the determination methodof transmission weight using the comparative example, the amount ofcalculation in the mobile station is increased undesirably because it isdesirable that the mobile station performs calculation using V1 in anyupdate of V1 and V2.

Next, a determination method using a comparative example of transmissionweight in a case in which MU-MIMO and SU-MIMO are present is describedwith reference to FIG. 5.

FIG. 5 is an illustrative diagram illustrating the determination methodusing a comparative example of transmission weight in the case in whichMU-MIMO and SU-MIMO are present. As illustrated in FIG. 5, in the casein which MU-MIMO and SU-MIMO are present, the base station and themobile station updates V2_SU for all a CSI_RS in addition to V1 andV2_MU. Therefore, calculation load in the mobile station is furtherincreased undesirably because the V2_SU is updated. However, in order torealize dynamic switching of MU-MIMO and SU-MIMO, it is important toevaluate both of the V2_MU and V2_SU all the time.

The above-described determination method of transmission weight by acomparative example is summarized as follows:

(1) Calculation Load in the Mobile Station is High

Reason: As described with reference to FIG. 4, calculation using alreadydetermined V1 is performed even in the case of evaluating V2_MU.

(2) Calculation Load in the Mobile Station is Further Increase WhenDynamic Switching of MU-MIMO and SU-MIMO is Tried to be Realized.

Reason: As described with reference to FIG. 5, both of the V2_MU andV2_SU are evaluated all the time.

In addition, when dynamic switching is performed in a communicationsystem using a plurality of subcarriers of an OFDM modulation scheme,etc., there has been no an allocation method of a frequency subcarrierthat can effectively reduce the amount of calculation.

Therefore, the embodiments of the present disclosure have been led tocreation by regarding the above circumstances as a point of view.According to each embodiment of the present disclosure, calculation loadin the mobile station 20 for determining transmission weight can besuppressed. Each of such embodiments of the present disclosure isdescribed below in detail.

2. Basic Configuration of a Base Station

A technology according to the present disclosure can be implemented invarious forms as described in detail in “4-1. First embodiment” to “4-7.Seventh embodiment” as examples. In addition, the base station 10according to each of the embodiments includes:

A: a communication unit (an antenna 110, an analog processing unit 120,etc.) that transmits a reference signal (CSI_RS),

B: a first multiplication unit (V1 multiplication unit 154) thatperforms multiplication of first transmission weight (V1) that isdetermined on the basis of reception of the reference signal by acommunication partner (the mobile station 20), and

C: a second multiplication unit (V2_MU multiplication unit 156) thatperforms multiplication of second transmission weight (V2_MU) that isdetermined on the basis of reception of the reference signal by thecommunication partner. In addition,

D: the communication unit transmits a reference signal with weight(V1*CSI_RS) obtained by multiplying the reference signal by the firsttransmission weight after determination of the first transmissionweight.

First, a common basic configuration in the base station 10 according tosuch embodiments is described below with reference to FIGS. 6 to 8.

FIG. 6 is an illustrative diagram illustrating a configuration of thebase station 10 according to the embodiment of the present disclosure.As illustrated in FIG. 6, the base station 10 according to theembodiment of the present disclosure includes the plurality of antennas110, a switch SW 116, an analog processing unit 120, an AD/DA conversionunit 124, a demodulation processing unit 128, an upper layer signalprocessing unit 132, a scheduler 136, a modulation processing unit 140,and a weight multiplication unit 150.

The antennas 110A to 110N function as a reception unit that converts aradio signal that is transmitted from the mobile station 20 into anelectrical reception signal and supplies the converted signal to theanalog processing unit 120, and a transmission unit that converts atransmission signal supplied from the analog processing unit 120 into aradio signal and transmits the converted signal to the mobile station20. Note that the number of the antennas 110 is not particularlylimited, and may be, for example, 8 or 16.

The switch SW 116 is a switch for switching a transmission operation anda reception operation by the base station 10. The base station 10performs the transmission operation when the antennas 110A to 110N areconnected to a transmission circuit of the analog processing unit 120through the switch SW 116, and performs the reception operation when theantennas 110A to 110N are connected to a reception circuit of the analogprocessing unit 120 through the switch SW 116.

The analog processing unit 120 includes the transmission circuit thatperforms analog processing for a transmission signal, and the receptioncircuit that performs analog processing for a reception signal. In thetransmission circuit, for example, up-conversion, filtering, gaincontrol, etc. of a transmission signal, in an analog form, which issupplied from the AD/DA conversion unit 124 are performed. In thereception circuit, for example, down-conversion, filtering, etc. of areception signal that is supplied from the antenna 110 through theswitch SW 116 are performed.

The AD/DA conversion unit 124 performs analogue/digital (AD) conversionof a reception signal that is supplied from the analog processing unit120, and performs digital/analogue (DA) conversion of a transmissionsignal that is supplied from the weight multiplication unit 150.

The demodulation processing unit 128 performs demodulation processing ofa reception signal that is supplied from the AD/DA conversion unit 124.The demodulation processing that is performed by the demodulationprocessing unit 128 may include OFDM demodulation processing, MIMOdemodulation processing, error correction, etc.

The upper layer signal processing unit 132 performs processing forinputting and outputting transmission data and reception data betweenthe upper layer signal processing unit 132 and an upper layer, controlprocessing of the scheduler 136, the modulation processing unit 140, andthe weight multiplication unit 150, determination processing of eachtransmission weight based on feedback information from the mobilestation 20, etc.

In addition, the base station 10 according to the embodiment transmits aV1*CSI_RS (reference signal with weight) obtained by multiplying aCSI_RS by V1 in addition to a CSI_RS (reference signal) afterdetermination of the transmission weight V1 on the basis of feedbackinformation from the mobile station 20 as described later in detail. Theupper layer signal processing unit 132 includes a function as areference signal management unit that manages a resource fortransmitting the CSI_RS and a V1*CSI_RS. In addition, the upper layersignal processing unit 132 controls the weight multiplication unit 150so that transmission of the CSI_RS or V1*CSI_RS is performed in theallocated resource.

The scheduler 136 allocates a resource for data communication to each ofthe mobile stations 20. The resource that is allocated by the scheduler136 is reported to each of the mobile stations 20 by a control channel,and each of the mobile stations 20 performs data communication in uplinkor downlink using the reported resource.

The modulation processing unit 140 performs modulation processing suchas mapping based on a constellation on transmission data that issupplied from the upper layer signal processing unit 132. Thetransmission signal obtained after modulation by the modulationprocessing unit 140 is supplied to the weight multiplication unit 150.

The weight multiplication unit 150 multiplies the transmission signalthat is supplied from the modulation processing unit 140 by thetransmission weight V1 and transmission weight V2_MU that are determinedby the upper layer signal processing unit 132 at the time of executionof MU-MIMO. On the other hand, the weight multiplication unit 150multiplies the transmission signal that is supplied from the modulationprocessing unit 140 by the transmission weight V2_SU that is determinedby the upper layer signal processing unit 132 at the time of executionof SU-MIMO. In addition, the weight multiplication unit 150 multiplies aCSI_RS by V1 in a resource that is allocated for transmission of aV1*CSI_RS (the “*” is complex multiplication) by the upper layer signalprocessing unit 132. Such configuration of the weight multiplicationunit 150 is described below in more detail with reference to FIG. 7.

FIG. 7 is an illustrative diagram illustrating a configuration of theweight multiplication unit 150. As illustrated in FIG. 7, the weightmultiplication unit 150 includes selectors 151, 157, and 158, a V2_SUmultiplication unit 152, the V1 multiplication unit 154, and the V2_MUmultiplication unit 156.

The selector 151 supplies a transmission signal that is supplied fromthe modulation processing unit 140 to the V2_MU multiplication unit 156or the V2_SU multiplication unit 152. More specifically, the selector151 supplies a transmission signal to the V2_MU multiplication unit 156when setting of MIMO is MU-MIMO, and supplies a transmission signal tothe V2_SU multiplication unit 152 when setting of MIMO is SU-MIMO.

The V2_SU multiplication unit 152 multiplies the transmission signalthat is supplied from the selector 151 by V2_SU that is determined bythe upper layer signal processing unit 132.

On the other hand, the V2_MU multiplication unit 156 multiplies thetransmission signal that is supplied from the selector 151 by V2_MU thatis determined by the upper layer signal processing unit 132. Inaddition, the V1 multiplication unit 154 multiplies the transmissionsignal that is multiplied by the V2_MU, by V1.

The selector 157 selectively outputs the multiplication result by the V1multiplication unit 154, or the multiplication result by the V2_SUmultiplication unit 152. More specifically, the selector 157 outputs themultiplication result by the V1 multiplication unit 154 when setting ofMIMO is MU-MIMO and outputs the multiplication result by the V2_SUmultiplication unit 152 when setting of MIMO is SU-MIMO.

A selector 158 supplies a CSI_RS to the former part or the latter partof the V1 multiplication unit 154. More specifically, the selector 158supplies a CSI_RS to the latter part of the V1 multiplication unit 154in a resource that is allocated for transmitting the CSI_RS. In thiscase, the base station 10 transmits a CSI_RS that is not multiplied byV1.

On the other hand, the selector 158 supplies a CSI_RS to the former partof the V1 multiplication unit 154 in a resource that is allocated fortransmitting a V1*CSI_RS. In this case, the base station 10 transmits aV1*CSI_RS because the CSI_RS is multiplied by V1 in the V1multiplication unit 154.

Note that, in FIG. 7, the example is described in which the V1multiplication unit 154 is arranged in the latter part of the V2multiplication unit 156, however, the configuration of the weightmultiplication unit 150 is not limited to such example. For example, asdescribed below with reference to FIG. 8, the V1 multiplication unit 154may be arranged in the former part of the V2 multiplication unit 156.

FIG. 8 is an illustrative diagram illustrating a configuration of aweight multiplication unit 150′ according to a variant. As illustratedin FIG. 8, the weight multiplication unit 150′ according to the variantincludes the selectors 151, 155, 157, and 159, the V2_SU multiplicationunit 152, the V1 multiplication unit 154, and the V2_MU multiplicationunit 156.

In the weight multiplication unit 150′ according to the variant, asillustrated in FIG. 8, the V1 multiplication unit 154 is arranged in theformer part of the V2_MU multiplication unit 156. In addition, in theweight multiplication unit 150′ according to the variant, the selector159 supplies a CSI_RS to the former part of the V1 multiplication unit154 or the latter part of the V2_MU multiplication unit 156.

More specifically, the selector 159 supplies a CSI_RS to the latter partof the V2_MU multiplication unit 156 in a resource that is allocated fortransmitting a CSI_RS. In this case, the base station 10 transmits aCSI_RS that is not multiplied by V1.

On the other hand, the selector 159 supplies a CSI_RS to the former partof the V1 multiplication unit 154 in a resource that is allocated fortransmitting a V1*CSI_RS. In this case, the CSI_RS is multiplied by V1in the V1 multiplication unit 154, and the V1*CSI_RS that is themultiplication result is supplied from the selector 155 to the selector157 so as to bypass the V2_MU multiplication unit 156. As a result, thebase station 10 transmits the V1*CSI_RS.

As described above, the base station 10 according to the embodimentstarts to transmit a V1*CSI_RS after determination of transmissionweight V1. By such configuration, calculation load of V2_MU, etc. in themobile station 20 that is described below can be suppressed.

3. Basic Configuration of a Mobile Station

FIG. 9 is an illustrative diagram illustrating a configuration of themobile station 20 according to the embodiment. As illustrated in FIG. 9,the mobile station 20 according to the embodiments includes a pluralityof antennas 210, a switch SW 216, an analog processing unit 220, anAD/DA conversion unit 224, a demodulation processing unit 228, an upperlayer signal processing unit 232, a modulation processing unit 240, achannel matrix obtaining unit 244, and a weight determination unit 248.

The antennas 210A and 210B function as a reception unit that converts aradio signal that is transmitted from the base station 10 into anelectrical reception signal and supplies the converted signal to theanalog processing unit 220, and function as transmission unit thatconverts a transmission signal that is supplied from the analogprocessing unit 220 into a radio signal and transmits the convertedsignal to the base station 10. Note that the number of antennas 210 isnot limited, and for example, may be four, or eight.

The switch SW 216 is a switch for switching a transmission operation anda reception operation of the mobile station 20. The mobile station 20performs the transmission operation when the antennas 210A and 210B areconnected to a transmission circuit of the analog processing unit 220through the switch SW 216, and the mobile station 20 performs thereception operation when the antennas 210A and 210B are connected to areception circuit of the analog processing unit 220 through the switchSW 216.

The analog processing unit 220 includes a transmission circuit thatperforms analog processing on a transmission signal and a receptioncircuit that performs analog processing on a reception signal. In thetransmission circuit, for example, up-conversion, filtering, gaincontrol, etc. of a transmission signal in an analog form, which issupplied from the AD/DA conversion unit 224 are performed. In thereception circuit, for example, down-conversion, filtering, etc. of areception signal that is supplied from the antenna 210 through theswitch SW 216 are performed.

The AD/DA conversion unit 224 performs AD conversion of a receptionsignal that is supplied from the analog processing unit 220 and performsDA conversion of a transmission signal that is supplied from themodulation processing unit 240.

The demodulation processing unit 228 performs demodulation processing ofa reception signal that is supplied from the AD/DA conversion unit 224.The demodulation processing that is performed by the demodulationprocessing unit 228 may include OFDM demodulation processing, MIMOdemodulation processing, and error correction.

The upper layer signal processing unit 232 performs processing forinputting and outputting transmission data and reception data betweenthe upper layer signal processing unit 232 and an upper layer. Inaddition, the upper layer signal processing unit 232 supplies feedbackinformation that indicates transmission weight that is determined by theweight determination unit 248 to the modulation processing unit 240, astransmission data.

The modulation processing unit 240 performs modulation processing suchas mapping based on a constellation on transmission data that issupplied from the upper layer signal processing unit 232. Thetransmission signal obtained after modulation by the modulationprocessing unit 240 is supplied to the AD/DA conversion unit 224.

The channel matrix obtaining unit 244 obtains a channel matrix H betweenthe base station 10 and the mobile station 20 when a CSI_RS is receivedfrom the base station 10.

The weight determination unit 248 determines transmission weight of V1,V2_MU, V2_SU, etc. on the basis of the channel matrix H obtained by thechannel matrix obtaining unit 244. Here, as described above withreference to FIG. 4, when V2_MU is updated on the basis of the channelmatrix H obtained from the CSI_RS, the mobile station according to acomparative example multiplies the channel matrix H by alreadydetermined V1 and evaluates optimal V2_MU for the channel matrix H thatis multiplied by the V1. Therefore, in the mobile station according tothe comparative example, calculation using V1 is performed even at thetime of update of V2_MU.

On the contrary, in the embodiment, after determination of V1, V1*CSI_RSthat is a CSI_RS multiplied by the V1 is received from the base station10. A channel matrix H that is obtained from a V1*CSI_RS by the channelmatrix obtaining unit 244 is already in a form of being multiplied byV1. Thus, the weight determination unit 248 can update V2_MU on thebasis of the channel matrix H that is obtained from the V1*CSI_RSwithout performing calculation using V1. As a result, calculation loadin the mobile station 20 for update of V2_MU can be significantlyreduced.

4. Description of Each of the Embodiments

The basic configurations of the base station 10 and the mobile station20 according to each of the embodiments of the present disclosure aredescribed above. Next, each of the embodiments of the present disclosureis described in detail.

4-1. First Embodiment

FIG. 10 is an illustrative diagram illustrating a first embodiment ofthe present disclosure. As illustrated in FIG. 10, the base station 10transmits a V1*CSI_RS to update (determine) V2_MU when V1 is determinedafter transmitting a CSI_RS. As described above, the mobile station 20that has received a V1*CSI_RS can evaluate optimal V2_MU withoutperforming calculation using V1.

In addition, the base station 10 transmits a CSI_RS to update V1 aftertransmitting a V1*CSI_RS multiple times. After that, the base station 10transmits a V1*CSI_RS to update V2_MU.

In FIG. 10, an example is described in which the update frequency of V2is about 4 to 5 times the update frequency of V1, however relationshipof update frequency is not limited to the example. In practice, it isconceivable that the update frequency of V1 is more than 10 times theupdate frequency of V2.

4-2. Second Embodiment

As described in the first embodiment, when the base station 10 transmitsa V1*CSI_RS, the mobile station 20 can evaluate optimal V2_MU withoutcalculation using V1. Here, in order to realize dynamic switching ofMU-MIMO and SU-MIMO, it is desirable that the mobile station 20 obtainsV2_SU. However, it is difficult for the mobile station 20 to evaluateV2_SU from the V1*CSI_RS.

Therefore, the upper layer signal processing unit 132 of the basestation 10 according to a second embodiment allocates a resource fortransmitting a CSI_RS to update (determine) V2_SU in addition toallocation of a resource for transmitting a V1*CSI_RS to update(determine) V2_MU. An operation of the base station 10 according to suchsecond embodiment is described in detail with reference to FIG. 11.

FIG. 11 is an illustrative diagram illustrating the second embodiment ofthe present disclosure. As illustrated in FIG. 11, the base station 10according to the second embodiment transmits a V1*CSI_RS to update V2_MUafter determination of V1, and transmits a CSI_RS to update (determine)V2_SU. By such configuration, the dynamic switching of MU-MIMO andSU-MIMO can be realized because V2_MU is obtained on the basis of theV1*CSI_RS and V2_SU is obtained on the basis of the CSI_RS.

Note that the mobile station 20 can determines that a radio signal thatis received from the base station 10 is a CSI_RS or a V1*CSI_RS, forexample, by a method that is described below.

(1) The base station 10 reports timing, order, etc. of transmission of aCSI_RS or a V1*CSI_RS through RRC signaling beforehand, to the mobilestation 20.

(2) The base station 10 reports timing, order, etc. of transmission of aCSI_RS or a V1*CSI_RS to the mobile station 20 by broadcasting systeminformation.

(3) The base station 10 transmits a CSI_RS and a V l*CSI_RS afterperforming addition of identification information that indicates aCSI_RS or a V1*CSI_RS.

4-3. Third Embodiment

As described in “1-4. Dynamic switching”, in SU-MIMO, for example, MIMOtransmission of eight independent streams is performed. On the otherhand, in MU-MIMO, for example, MIMO transmission of two independentstreams is performed for each of the four different mobile stations 20.Thus, V2_SU and V2_MU are different in terms that V2_SU is used foreight streams and V2_MU is used for two streams.

In this case, it is effective to set update frequency of V2_SU higherthan update frequency of V2_MU because higher accuracy is desired forV2_SU that is used for eight streams.

Therefore, the upper layer signal processing unit 132 of the basestation 10 according to a third embodiment allocates more resources fortransmitting a CSI_RS to update (determine) V2_SU than that fortransmitting of a V1*CSI_RS to update (determine) V2_MU. An operation ofthe base station 10 according to such third embodiment is described indetail with reference to FIG. 12.

FIG. 12 is an illustrative diagram illustrating the third embodiment ofthe present disclosure. As illustrated in FIG. 12, the base station 10according to the third embodiment transmits, on a time direction, aCSI_RS to update (determines) V2_SU at higher frequency than that of aV1*CSI_RS to update (determines) V2_MU after determination of V1. Bysuch configuration, highly accurate V2_SU can be obtained whilesuppressing calculation load in the mobile station 20 at the time ofupdate of V2_MU.

4-4. Fourth Embodiment

In the third embodiment, the description is made in which the basestation 10 transmits, in the time direction, a CSI_RS at higherfrequency than that of a V1*CSI_RS in order to make the update frequencyof V2_SU higher than the update frequency of V2_MU. In a fourthembodiment, similarly to the third embodiment, arrangement of aV1*CSI_RS and a CSI_RS on the frequency direction in a subcarrier ofOFDM has been devised in order to make the update frequency of V2_SUhigher than the update frequency of V2_MU. A resource allocation exampleaccording to the fourth embodiment is described below in detail withreference to FIG. 13.

FIG. 13 is an illustrative diagram illustrating a resource allocationexample of a V1*CSI_RS and a CSI_RS according to the fourth embodiment.As illustrated in FIG. 13, the upper layer signal processing unit 132 ofthe base station 10 according to fourth embodiment arranges, on thefrequency direction, a CSI_RS more densely than a V1*CSI_RS. Asdescribed above, similarly to the third embodiment, highly accurateV2_SU can be obtained while suppressing calculation load in the mobilestation 20 at the time of update of V2_MU, by devising the arrangementof a V1*CSI_RS and a CSI_RS on the frequency direction.

4-5. Fifth Embodiment

In a fifth embodiment, resource allocation for data communication usinga determined transmission weight is described.

FIG. 14 is an illustrative diagram illustrating a specific example ofresource allocation according to the fifth embodiment. The horizontalaxis in FIG. 14 indicates a time, and the vertical axis indicates afrequency. In addition, the time width of a square block in FIG. 14 maybe one resource block or one subframe. In addition, the frequency widthof the square block may be one resource block (12 subcarrier portions)or another band width.

As illustrated in FIG. 14, when the base station 10 transmits a CSI_RSfirst, the mobile station 20 obtains V1, V2_MU, and V2_SU for eachfrequency on the basis of the reception of a CSI_RS. In addition, themobile station 20 provides feedback of V1, V2_MU, and V2_SU to the basestation 10.

After that, as illustrated in FIG. 14, the scheduler 136 of the basestation 10 allocates four resource blocks from the bottom included in afrequency range B for MU-MIMO (first scheme) with the mobile stations20A to 20C. On the other hand, as illustrated in FIG. 14, the scheduler136 of the base station 10 allocates two resource blocks from the topincluded in a frequency range A for SU-MIMO (second scheme) with themobile station 20D.

Here, the scheduler 136 according to the fifth embodiment keeps theresource blocks that are included in the frequency range B as an areafor MU-MIMO and keeps the resource blocks that are included in thefrequency range A as an area for SU-MIMO.

Therefore, for example, when the scheduler 136 according to the fifthembodiment performs dynamic switching of setting of MIMO of the mobilestation 20C from MU-MIMO to SU-MIMO, the resource block that isallocated to the mobile station 20C is moved to the resource block thatis included in the frequency range A, as illustrated in FIG. 14.

As described above, according to the fifth embodiment, dynamic switchingof MU-MIMO and SU-MIMO can be realized by moving a resource block of themobile station 20 in a frequency direction.

4-6. Sixth Embodiment

FIG. 15 is an illustrative diagram illustrating a specific example ofresource allocation according to a sixth embodiment. As illustrated inFIG. 15, the upper layer signal processing unit 132 according to thesixth embodiment allocates resource blocks that are included in thefrequency range B for MU-MIMO that is described in the fifth embodiment,for transmitting a V1*CSI_RS, after determination of V1. In addition,the upper layer signal processing unit 132 allocates resource blocksthat are included in the frequency range A for SU-MIMO that is describedin the fifth embodiment, for transmitting a CSI_RS.

By such configuration, V2_SU can be updated in frequency range A whilesuppressing the amount of calculation in the mobile station 20 andupdating V2_MU in the frequency range B. Therefore, the frequency rangeB can be used for communication by MU-MIMO, and the frequency range Acan be used for communication by SU-MIMO.

4-7. Seventh Embodiment

In the above-described fifth embodiment and sixth embodiment, theexample is described in which a frequency range for MU-MIMO and afrequency range for SU-MIMO are fixed, and alternatively, as describedbelow with reference to a seventh embodiment, a frequency range forMU-MIMO and a frequency range for SU-MIMO can be dynamically changed.

FIG. 16 is an illustrative diagram illustrating a specific example ofresource allocation according to a seventh embodiment. As illustrated inFIG. 16, it is assumed that, in the time t1, resource blocks in afrequency range Z and a frequency range X are allocated for transmittinga CSI_RS, resource blocks in a frequency range Y are allocated fortransmission of a V1*CSI_RS.

Here, in a frequency in which a CSI_RS is transmitted, V1, V2_MU, andV2_SU can be obtained. On the other hand, in a frequency in which aV1*CSI_RS is transmitted, V2_MU can be obtained, however, V2_SU isdifficult to be obtained. That is, the frequency in which a V1*CSI_RS istransmitted can be used for MU-MIMO, the frequency in which a CSI_RS istransmitted can be used for both of MU-MIMO or SU-MIMO.

Therefore, the scheduler 136 according to the seventh embodiment handlesresource blocks in the frequency range X and the frequency range Z towhich a CSI_RS is transmitted as an area in which switching of SU-MIMOand MU-MIMO can be performed. On the other hand, the scheduler 136handles resource blocks in the frequency range Y to which a V1*CSI_RS istransmitted as a MU-MIMO-dedicated area.

For example, as illustrated in FIG. 16, at the time t2, the scheduler136 allocates resource blocks in the frequency range X for communicationby SU-MIMO, and allocates resource blocks in the frequency range Y andthe frequency Z for communication by MU-MIMO. After that, at the timet3, the scheduler 136 can switches resource blocks for MU-MIMO toresource blocks for SU-MIMO in the frequency range Z, and can switchesat resource blocks for SU-MIMO to at resource blocks for MU-MIMO in thefrequency range X.

5. Operation of the Base Station and the Mobile Station

Each of the embodiments of the present disclosure is described above.Next, operations of the base station 10 and the mobile station 20according to the embodiments of the present disclosure are describedwith reference to FIGS. 17 and 18.

FIG. 17 is a flowchart illustrating an operation of the base station 10according to the embodiments of the present disclosure. Note that FIG.17 particularly corresponds to the operation of the base station 10according to the seventh embodiment.

As illustrated in FIG. 17, first, the base station 10 determines updatefrequency of V1 and update frequency of V2_MU in the time direction(S304). After that, the base station 10 determines update frequency ofV2_SU in the time direction (S308).

After that, the base station 10 determines a density of a resource forMU-MIMO and a density of a resource for SU-MIMO in the frequencydirection (S312). In addition, the base station 10 determines a ratio,which is illustrated in FIG. 16, of the MU-MIMO-dedicated area and thearea in which dynamic switching can be performed in the frequencydirection (S316). Note that in the example illustrated in FIG. 16, theratio of the MU-MIMO-dedicated area and the in which dynamic switchingcan be performed in the frequency direction is 1:2, and a density ratioof a resource for MU-MIMO and a resource for SU-MIMO in the frequencydirection is 2:1.

After that, the base station 10 allocates a resource for transmitting aCSI_RS and a resource for transmitting a V1*CSI_RS (S320). Morespecifically, in S316, the base station 10 allocates a resource of afrequency that is determined as the MU-MIMO-dedicated area fortransmitting a V1*CSI_RS and allocates a resource of a frequency that isdetermined as the area in which dynamic switching can be performed fortransmitting a CSI_RS. In addition, the base station 10 allocates aresource in the time direction to a V1*CSI_RS and a CSI_RS on the basisof the determination results of S304 and S308. In addition, the basestation 10 transmits a CSI_RS and a V1*CSI_RS in accordance with thedetermined resource.

FIG. 18 is a flowchart of an operation of the mobile station 20according to the embodiments. As illustrated in FIG. 18, in a case inwhich the mobile station 20 receives a radio signal from the basestation 10 (S404), when the radio signal is a CSI_RS (S408), a channelmatrix H is obtained from the reception result of the CSI_RS (S412). Inaddition, the mobile station 20 determines transmission weight such asV1, V2_MU, and V2_SU on the basis of the channel matrix H obtained inS412 (S416). In addition, the mobile station 20 provides feedback of V1,V2_MU, and V2_SU to the base station 10 (S420).

On the other hand, when the received radio signal is a V1*CSI_RS (S408),the mobile station 20 obtains a channel matrix H that is multiplied byV1 from the reception result of a V1*CSI_RS (S424). In addition, themobile station 20 determines V2_MU on the basis of the channel matrix Hthat is multiplied by V1 without performing calculation using V1 (S428).In addition, the mobile station 20 provides feedback of V2_MU to thebase station 10 (S432).

In addition, when the received radio signal is a data signal (S408), themobile station 20 demodulates the data signal and obtains data that istransmitted from the base station 10 (S436).

6. Conclusion

As described above, the base station 10 according to the embodiments ofthe present disclosure starts to transmit a V1*CSI_RS afterdetermination of transmission weight V1. By such configuration,calculation load such as V2_MU in the mobile station 20 that isdescribed below can be suppressed. In addition, the base station 10according to the embodiments of the present disclosure continues totransmit a CSI_RS. By such configuration, the mobile station 20 candetermine V2_SU on the basis of reception of a CSI_RS. As a result,dynamic switching of MU-MIMO and SU-MIMO can be realized.

The preferred embodiments of the present invention have been describedabove with reference to the accompanying drawings, whilst the presentinvention is not limited to the above examples, of course. A personskilled in the art may find various alternations and modificationswithin the scope of the appended claims, and it should be understoodthat they will naturally come under the technical scope of the presentinvention.

For example, two or more of the first embodiment to the seventhembodiment may be combined. For example, the resource allocation in thetime direction that is described in the third embodiment, the resourceallocation in the frequency direction that is described in the fifthembodiment, and the resource allocation for SU-MIMO and MU-MIMO that isdescribed in the sixth embodiment can be combined.

In addition, the steps in the processing of the base station 10 or theprocessing of the mobile station 20 in this specification are notnecessarily processed in chronological order in accordance with theorder that is described as the flowchart. For example, the steps in theprocessing of the base station 10 or the processing of the mobilestation 20 may be processed in order different from the order that isdescribed as the flowchart, or may be processed in parallel.

In addition, a computer program can be created that exerts hardware suchas a CPU, a ROM, and a RAM, which is built in the base station 10 or themobile station 20 as a function equivalent to each configuration of theabove-described base station 10 or the mobile station 20. In addition, astorage medium that stores the computer program is also provided.

REFERENCE SIGN LIST

-   10 base station-   20, 20A, 20B mobile station-   110, 210 antenna-   116, 216 switch SW-   120, 220 analog processing unit-   124, 224 AD/DA conversion unit-   128, 228 demodulation processing unit-   132, 232 upper layer signal processing unit-   136 scheduler-   140, 240 modulation processing unit-   150 weight multiplication unit-   152 V2_SU multiplication unit-   154 V1 multiplication unit-   156 V2_MU multiplication unit-   244 channel matrix obtaining unit-   248 weight determination unit

1. A wireless communication device comprising: a communication unit thattransmits a reference signal; a first multiplication unit that performsmultiplication of first transmission weight that is determined based onreception of the reference signal by a communication partner; and asecond multiplication unit that performs multiplication of secondtransmission weight that is determined based on reception of thereference signal by the communication partner, wherein the communicationunit transmits a reference signal with weight that is obtained bymultiplying the reference signal by the first transmission weight afterdetermination of the first transmission weight.
 2. The wirelesscommunication device according to claim 1, further comprising: areference signal management unit that manages a resource fortransmitting the reference signal with weight.
 3. The wirelesscommunication device according to claim 2, wherein the reference signalmanagement unit allocates a resource for transmitting the referencesignal with weight and a resource for transmitting the reference signalafter determination of the first transmission weight.
 4. The wirelesscommunication device according to claim 3, wherein the reference signalmanagement unit allocates more resources for transmitting the referencesignal than the resource for transmitting the reference signal withweight.
 5. The wireless communication device according to claim 4,wherein the reference signal management unit allocates a resource sothat transmission frequency of the reference signal on a time axisbecomes higher than transmission frequency of the reference signal withweight on the time axis.
 6. The wireless communication device accordingto claim 4, wherein the reference signal management unit allocates aresource so that a density of a resource for transmitting the referencesignal on a frequency axis becomes higher than a density of a resourcefor transmitting the reference signal with weight on the frequency axis.7. The wireless communication device according to claim 3, furthercomprising: a scheduler that allocates a resource for communication of afirst scheme or a second scheme to each communication partner, andwherein the scheduler allocates a resource within a first frequencyrange for the communication of the first scheme, and allocates aresource within a second frequency range for the communication of thesecond scheme.
 8. The wireless communication device according to claim7, wherein the first frequency range is a frequency range to which aresource for transmitting the reference signal with weight is allocated,and wherein the second frequency range is a frequency range to which aresource for transmitting the reference signal is allocated.
 9. Thewireless communication device according to claim 8, wherein the firstscheme is multi user multi input multi output (MU-MIMO), the secondscheme is single user multi input multi output (SU-MIMO).
 10. Thewireless communication device according to claim 3, further comprising:a scheduler that allocates a resource for communication of a firstscheme or a second scheme to each communication partner, wherein, thescheduler allocates, for the communication of the first scheme, aresource within a frequency range to which a resource for transmittingthe reference signal with weight is allocated, and allocates, for thecommunication of the first scheme or the second scheme, a resourcewithin a frequency range to which a resource for transmitting thereference signal is allocated.
 11. The wireless communication deviceaccording to claim 3, wherein update frequency of the secondtransmission weight is higher than update frequency of the firsttransmission weight.
 12. The wireless communication device according toclaim 11, wherein the first transmission weight is weight for formingdirectivity, and the second transmission weight is non-directionalweight for adjusting a phase.
 13. A program for causing a computer tofunction as a wireless communication device that includes: acommunication unit that transmits a reference signal; a firstmultiplication unit that performs multiplication of first transmissionweight that is determined based on reception of the reference signal bya communication partner; and a second multiplication unit that performsmultiplication of second transmission weight that is determined based onreception of the reference signal by the communication partner, andwherein the communication unit transmits a reference signal with weightthat is obtained by multiplying the reference signal by the firsttransmission weight after determination of the first transmissionweight.
 14. A wireless communication method comprising: transmitting areference signal; multiplying the reference signal by first transmissionweight that is determined based on reception of the reference signal bya communication partner; and transmitting a reference signal with weightthat is obtained by multiplying the reference signal by the firsttransmission weight.
 15. A wireless communication system comprising: afirst wireless communication device; and a second wireless communicationdevice that includes, a communication unit that transmits a referencesignal, a first multiplication unit that performs multiplication offirst transmission weight that is determined based on reception of thereference signal by the first wireless communication device, and asecond multiplication unit that performs multiplication of secondtransmission weight that is determined based on reception of thereference signal by the first wireless communication device, and whereinthe communication unit transmits a reference signal with weight that isobtained by multiplying the reference signal by the first transmissionweight after determination of the first transmission weight.
 16. Awireless communication device comprising: a communication unit thatreceives a reference signal from a communication partner; and a weightdetermination unit that determines first transmission weight and secondtransmission weight based on a reception result of the reference signalby the communication unit, and wherein when a reference signal withweight that is obtained by multiplying the reference signal by the firsttransmission weight is received by the communication unit, the weightdetermination unit determines the second transmission weight based on areception result of the reference signal with weight.
 17. A wirelesscommunication device comprising: a scheduler that allocates a resourcefor communication of a first scheme or a second scheme to eachcommunication partner, wherein the scheduler allocates a resource withina first frequency range for the communication of the first scheme, andallocates a resource within a second frequency range for thecommunication of the second scheme.
 18. The wireless communicationdevice according to claim 17, wherein the first scheme is multi usermulti input multi output (MU-MIMO), and the second scheme is single usermulti input multi output (SU-MIMO).